This invention relates to a shadow mask type color picture tube and, more particularly to the shadow mask supporting structure.
For supporting a shadow mask in a color picture tube, the method of suspension by panel stud pins embedded at the diagonals of the panel inner wall and using spring members is already known.
For example, Japanese Patent Publication No. 46-4104 describes the construction of a shadow mask which is secured at the four corners of the roughly oblong-shaped or rectangular panel window by spring members. When the spring members described in the Publication are used, as shown in FIG. 3, the shadow mask 8 moves slightly towards a phosphor screen 5 through expansion due to rise in temperature. A shift of electron beam 4 also occurs caused by the relative displacement between the shadow mask aperture and the relevant phosphor dots on the screen due to this expansion, so color reproduction error being corrected.
When a spring member is designed based on the above correction principle, it is shown in FIG. 4 which is described in U.S. Pat. No. 4,300,071 (Dougherty). It is a spring member 10 which is constructed by bending a metal strip which is secured at one end to a frame 9, which supports a shadow mask 8, or to a rigid support secured to that frame and at the other end is secured to a panel stud pin 2. However, this construction has a fault in that, when the mounting and dismounting of spring member 10 is repeated several times and it is bent as shown by broken line 10a during the process of making a color picture tube, plastic deformation occurs at bend 10, and it does not recover its original shape.
In addition, since four spring members are used to support one shadow mask and, they are mass-produced, it is impossible to make a completely homogeneous set. Mounting on the stud pins more than once and the heating to which they are subjected during the color picture tube manufacturing process causes the differences of randomness in the properties of these spring members to increase, so some members occuring unexpectedly plastic deformation. When the deformation of one of the four spring members secured to the shadow mask is greater than that of the others, the shadow mask is displaced from the desired position. As a result, the paths of the electron beams which pass through the shadow mask are shifted from the phosphor dots and the white uniformity deteriorates. Even if the best stainless steel (SUS631) is used for the spring members of a color picture tube, this defect can not be overcome.
As a modification to this, a spring member 20 which is bent in a dog-leg shape from the upper end of the plate portion which is welded to the frame 9 towards the stud pin 2 below, as shown in FIG. 5 can be considered. In this form, when it is bent as at a broken line position 20a for mounting or dismounting, since the wole of the upper portion bends from welding points 13, irrecoverable permanent deformation can be avoided. However, it has been proved that spring members which are formed by this kind of process of bending a continuous spring member have the fault of weakness to external mechanical shock. As shown in FIG. 6, when a force F.sub.1 from the panel stud pin (not shown) acts on the circumference of the hole 21 which accommodates the panel stud pin, and a force F.sub.2 acts in the opposite direction to force F.sub.1 on the surface 20.sub.3 which is welded to the frame (not illustrated), permanent deformation of the spring member 23 can easily occur. This type of couple can sometimes occur when transporting color picture tubes.
When the permanent deformation was studied, it was found that the deformation of both ends 20.sub.1 and 20.sub.2 of the bend of spring member 20 due to the couple was marked. When a couple in the direction shown in FIG. 6 was applied to spring member 20, the deformation shown by the broken line 20.sub.4 in FIG. 7 occurred. The reason for this was proved to be that the arc portion shown by arrow 20.sub.5 in FIG. 8 is liable to bend, i.e., when seen in FIG. 7, the radius of the arc at the end 20.sub.1 becomes smaller and the radius at end 20.sub.2 becomes greater.